Vehicles powered by liquid propane are increasingly replacing vehicles powered by gasoline or diesel fuel. Primary reasons for this conversion to propane powered vehicles are reduced maintenance costs for operating the vehicles, and reduced emission of harmful contaminants from the vehicle engine. This latter feature is particularly important for forklifts, sweepers, carts and other vehicles which frequently operate within a warehouse, manufacturing plant, or other substantially enclosed buildings. Reduced emission of contaminants from vehicle engines powered by propane is essential to the satisfactory performance of many vehicles. Clean running engines powered by liquid propane deliver the desired power output to the forklift operator, and do not create a harmful environment within the building. Accordingly, forklifts powered by liquid propane are preferred in many applications over battery powered forklifts.
The primary contaminant of concern in the engine exhaust streams of a propane powered vehicle is carbon monoxide (CO). Due to the hazardous nature of this gas, OSHA guidelines strictly regulate the permissible CO level in a workplace. Vehicles operating in a fuel rich mode are a primary generator of carbon monoxide within some workplace environments. Accordingly, various systems have been devised which seek to avoid the fuel rich engine operation mode and maintain the stoichiometric operation of a propane powered engine, thereby effectively limiting the discharge of carbon monoxide from the engine. Those skilled in the industry recognize that a propane powered engine operating at its stoichiometric point releases a small fraction of the carbon monoxide discharged from an engine operating in a fuel rich mode.
A new propane powered vehicle engine is set to operate at its stoichiometric point, but engine wear and use alters the ideal air-to-fuel ratio. Techniques for limiting the carbon monoxide discharge from propane powered vehicles include the periodic check of engine exhaust, and the regular maintenance of engines to insure an acceptable discharge of contaminants from the vehicle. Although such systems are still widely used today, it is difficult to insure acceptable carbon monoxide discharge levels from a vehicle using this technique alone, since vehicle operation and/or maintenance personnel may not regularly and properly maintain stoichiometric engine operation. Moreover, an engine running fuel rich provides the maximum power output, and thus is understandably preferred by the vehicle operator for maximum performance. Although a fuel lean engine mode produces an acceptable carbon monoxide output, engine performance suffers when the engine runs too lean. Accordingly, a slightly fuel lean or stoichiometric engine operation is preferred to both achieve acceptable vehicle performance and insure safe discharge levels of carbon monoxide in the engine exhaust stream.
In an effort to reduce the problems associated with carbon monoxide discharged from a propane powered vehicle engine, various companies have marketed "closed-loop" systems which automatically control the air-to-fuel ratio of an engine in response to the detected oxygen concentration in the exhaust stream, and thereby seek to control the carbon monoxide output. Existing systems utilize a sensor mounted on the vehicle to continually detect oxygen levels in the engine discharge stream. Detected oxygen correlates well with the carbon monoxide concentration in the discharge stream, and reliable oxygen sensors which will not saturate at engine startup cost substantially less than reliable carbon monoxide sensors. These systems seek to maintain the engine in a slightly fuel lean or stoichiometric mode to limit the CO discharge from the vehicle. Closed-loop systems typically include bypass mechanisms to allow the engine to run fuel rich at startup.
Closed-loop systems are expensive, and many systems can be tampered with by vehicle operators. Equally important, the ability of a closed-loop system to effect the carbon monoxide discharge by adjusting the air-to-fuel ratio is limited. If a propane powered vehicle drifts out of the range of the control exercisable by the closed-loop system, a warning light may come on to alert the operator. This light often is ignored by the vehicle operator for reasons explained above. The warning light may also burn out. Other warning systems may be ignored or bypassed. Accordingly, high workplace carbon monoxide levels result in unnecessary exposure to employees within buildings where propane powered vehicles are operating, and subject many employers to numerous health related claims.
The disadvantages of the prior art are overcome by the present invention, and an improved system and method are hereinafter disclosed for reliably controlling the operation of a vehicle engine to reduce the discharge of harmful contaminants. The system and method of the present invention are particularly well suited for requiring the operation of a propane powered engine in a slightly lean or stoichiometric mode.